Can someone please help me with this?

This is from a previous years math competition. Matemaattisten Aineiden Opettajien Liitto MAOL ry : Tehtäviä

If an empty bath is plugged and the tap opened, the tub fills in a minutes. if the plug is removed the bath empties in b minutes , b>a. How long will it take for the bath to fill if the tap is opened and the plug taken away?

Can you please also explain how one "begins" to solve a word problem as such?

Thank you!